Electrostatography broadly includes the forming and developing of electrostatic image patterns either with or without light exposure through the use of an electrostatically charged toner composition. It includes electrophotography, as used, for example, in office copying machines, as well as processes such as dielectric recording which require no light exposure.
A principal type of electrostatically chargeable toner composition for such processes is a dry fusible powder composed mainly of a thermoplastic, i.e., polymer, binder resin and a colorant such as carbon black or an organic pigment or dye. This kind of dry toner powder is normally used in a developer composition in admixture with carrier particles (usually larger in size than the toner particles). These triboelectrically charge the toner particles so that they can be electrostatically attracted to oppositely charged areas of an electrostatic latent image to develop the image, the toner thereafter being transferred to and fixed by fusion or other means to an image receiving sheet, e.g., a sheet of paper.
The facilitation of pigment dispersion in polymers is an ongoing problem in the manufacturing of electrostatographic toners since the dispersion quality of pigment in toner affects toner performance in several ways. Typically, as the pigment dispersion quality improves, the spectral density (Dmax) of the toner increases and the amount of unwanted spectral absorption of the toner decreases, resulting in an improved system color gamut. Further, as the pigment dispersion quality improves, the amount of pigment needed in the toner decreases resulting in a decrease in the cost of the toner. The toner also becomes more uniform as the pigment dispersion quality improves, and this uniformity is desirable to ensure optimum performance. Therefore, improved dispersion of pigments is desired to increase the variety of colors available for use in color copiers. Most pigments are formed via a precipitation process. The precipitation step, as well as subsequent purification and isolation steps, can lead to agglomeration thereby necessitating the dispersion of the pigment in the product to which it is applied. Currently, pigments that are difficult to disperse are either purchased as flushes or compounded at high pigment concentrations with polymers. Neither of these techniques produce the desired degree of pigment dispersion.
The desired degree of pigment dispersion can be achieved by milling in solvents that reduce pigment particle cohesion. Alternatively, the pigment in its precipitation or purification liquor may be used if no agglomeration has occurred. The resulting dispersion can be metered into extruders capable of handling liquid feed. Since most of the feed stock to an extruder produces toner solids, two separate feeds must be maintained in an exact ratio to obtain the desired pigment loading if solvent mill pigment is to be fed directly to the extruder. Furthermore, the milling of pigments in solvents causes problems in storage and addition of milled pigments to compounders for conventional toners. Further, pigment suspensions have a limited shelf life and require ventilated storage. Although solutions and suspensions can be added to extruder type compounders, modification of the building support is needed to accommodate the health and environmental hazards of solvent vapors.
The pigment dispersion may be "let down," i.e., diluted in a manner that preserves the degree of dispersion, in a polymer solution and dried to obtain a pigment concentrate with a long shelf life. However, pan drying of the solution/dispersion requires long drying times, high heats, and produces very thin films of material necessitating comminution before use. Also, a residual level of solvent in pan-dried pigment concentrate is difficult to remove by drying and the modifications to the building support service described above may still be needed. Other techniques such as drum driers may be used to automate pan drying but have the same drawbacks as described above.
Dry or particulate toner powders in which the pigment is dispersed can be prepared by a variety of techniques. One technique that takes advantage of milling pigments in solvent to obtain high dispersion qualities is limited coalescence. Limited coalescence toners are formed by dispersing an oil or organic phase (which contains a pigment or dye) in an aqueous media in the presence of colloidal stabilizers. The purpose of the stabilizer is to control the size by preventing the coalescence of the emulsion once the required size has been attained. Therefore, the size of the colorant particle formed by this technique is related to the amount of colloidal stabilizer, e.g., silica, present in the aqueous phase. The monomer is polymerized to form solid colorant containing polymeric particles that are separated and dried. The process is only useful (in practice) for making very small particle size toner powders (under about 15 microns) that can be employed in making high resolution developed toner images. The suspension limited coalescence process is described, for example, in U.S. Pat. Nos. 2,932,629 and 4,835,084.
Another example is an evaporation limited coalescence process, where the stabilizer used is also a colloidal silica, or the like, and where the suspended small droplets comprises a solution of polymer in a non-aqueous, water immiscible solvent liquid. The solvent is removed by evaporation and the particles are separated, washed and dried. Such a process is disclosed in U.S. Pat. No. 4,833,060 to Nair et al. ("Nair"). The limited coalescence technique described in Nair involves first forming a solution of a polymer in a solvent that is immiscible with water (along with any required addenda), and then suspending the polymer-solvent solution in water containing a promoter and a colloidal suspension stabilizer. The resulting suspension is subjected to high shear action to reduce the size of the polymer-solvent droplets. The shearing action is removed and the polymer-solvent droplets, with particles of stabilizer at their surface, coalesce to the extent allowed by the stabilizer to form coalesced polymer-solvent droplets. After the drop size has been determined by the limited coalescence, the solvent must be removed from the coalesced drops to form the solidified polymer particles which are then isolated from the suspension by filtration or other suitable means.
The Nair technique is referred herein to as evaporative limited coalescence ("ELC") whereby the particles are formed from a water immiscible solution or dispersion. In an "inverse" limited coalescence process, the particles are formed from a water or water-miscible solution or dispersion suspended in an immiscible hydrophobic liquid containing a colloidal suspension stabilizer. This inverse method is useful for producing particles having a narrow size distribution and controlled morphology from hydrophilic materials such as precursor to ferrites, ceramic powders, or water soluble polymers. Both techniques are useful to obtain powders with desirable handling properties (e.g., high powder flow, low dusting, etc.). Inverse limited coalescence is described in U.S. patent application Ser. Nos. 07/888,063 and 08/309,431, to K. D. Lofftus, both entitled "INVERSE LIMITED COALESCENCE PROCESSES", filed May 26, 1992 and Sep. 20, 1994, respectively.
A disadvantage, however, of solvent removal by evaporation is that evaporation of the solvent or dispersant, is a slow, time consuming process. In addition, many potential ELC systems are not sufficiently robust to withstand the stirring and heating that is required to complete the evaporation process and solidify the coalesced droplets. The agitation and heat destabilize the coalesced droplets before they are solidified, causing breakdown of the stabilized interface and agglomeration of the particles.
Solutions of polymer containing dispersed pigments can be formed into controlled particles using ELC while avoiding the comminution step (see U.S. Pat. Ser. No. 4,833,060). However, in practice, ELC is only able to produce toner particle sizes which dust badly and do not blend well with polymers before compounding. To solve this deficiency, a solvent extraction limited coalescence ("SXLC") technique as described in co-pending U.S. patent application Ser. No. 07/888,064 to K. D. Lofftus, filed May 26, 1992, was developed to rapidly remove the solvent and provide larger particles.
Larger particles made by SXLC contain less of the colloidal stabilizer which may act as a contaminant in later processes. In ELC toner production, it was found that colloidal silica stabilizer had to be removed, but latex stabilizers could be left on the particle surface. However, polymers made by polymerization limited coalescence ("PLC") can be larger in size and it has been found that the level of silica contamination does not adversely affect toners made by compounding these polymers with the silica in place. U.S. Pat. Ser. Nos. 4,912,009 and 4,965,131 are representative of polymerization limited coalescence methods employed in toner manufacture.
It is clear that there remains a need to provide a method of producing a dry form of pigment dispersion and a novel powder form of pigment which overcome the above-described deficiencies in the art.